Coherent laser arrays are capable of delivering significantly higher brightness than uncoupled arrays due to the significantly narrower far-field characteristics resulting from constructive interference of the individual emitters. Techniques for spectrally and spatially combining the outputs of individual elements of semiconductor laser arrays have been heavily researched, as artisans work toward the ultimate goal of producing high-power laser systems with high beam quality. Methods for achieving this have ranged from closely packing single-mode fiber lasers into an array, to the use of microlenses and diffraction gratings in order to coherently couple multiple semiconductor lasers. Spatial beam combination techniques have the advantage of operating at a single output frequency and are ideal for pumping applications such as for Er-doped fiber amplifiers.
Coupled semiconductor laser arrays have been studied for at least the last 20 years. Many methods of providing coherent coupling among elements of the array use external lenses and mirrors to provide feedback between adjacent elements. See, e.g., Leger, et. al., “Coherent addition of AlGaAs lasers using microlenses and diffractive coupling,” Applied Physics Letters, vol. 52, pp. 1771-1773, 1988. While this technique has been demonstrated to work, the fabrication is complicated by the need for several discrete components and precision alignment between these components must be achieved. Another technique for obtaining coupled output arrays from semiconductor laser diodes is the use of gratings which couple light out perpendicular to the plane of the wafer. An example is work concerning two-dimensional arrays of surface emitters that demonstrated coherent emission. Evans, et. al., “Coherent, monolithic two-dimensional strained InGaAs/AlGaAs quantum well laser arrays using grating surface emission,” Applied Physics Letters, vol. 55, pp. 2721-2723, 1989. Loss mechanisms associated with the gratings reduce achievable power output and device reliability. Fabrication is also relatively complex.
Fiber laser arrays can also be used to coherently combine the output of several discrete lasers. See, Cheo, et. al., “A high-brightness laser beam from a phase-locked multicore Yb-doped fiber laser array,” Photonics Technology Letters, IEEE, vol. 13, pp. 439-441, 2001. Their system combined the output of several fiber lasers in a two dimensional array. This configuration is able to achieve in phase coupling, resulting in a high quality output beam. There are, however, several disadvantages to using fiber lasers over using semiconductor diode lasers. First of all, there is an inherent inefficiency due to the fact that the fiber laser must be pumped by another optical source, typically a diode laser. Assembly of the fiber laser system is also significantly more complicated than the fabrication monolithically integrated lasers.
Combining of individual elements of semiconductor laser diodes has been attempted using several different techniques in recent years. See, Fan “Laser beam combining for high-power, high-radiance sources,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 11, pp. 567-577, 2005. An array of parabolic bow-tie laser is another structure that has been employed. Causa et al., “Observation and Analysis of Phase-Locking in Parabolic Bow-Tie Laser Arrays,” IEEE Journal of Quantum Electronics, vol. 42, pp. 1016-1022, 2006. Under certain operating conditions, these devices achieve in-phase coupling. Under other conditions, however, the devices demonstrate out of phase coupling, indicating a certain level of instability in the coupling characteristics of the individual devices. This type of behavior is typical of evanescently coupled semiconductor laser arrays. Y-junction coupled semiconductor laser arrays have also been used to achieve in-phase coupled high-brightness arrays. See, Streifer et al., “Y-Junction semiconductor laser arrays: Part I—Theory,” IEEE Journal of Quantum Electronics, vol. 23, pp. 744-751, 1987; Welch et al, “Y-Junction semiconductor laser arrays: Part II—Experiments,” IEEE Journal of Quantum Electronics, vol. 23, pp. 752-756, 1987. In this method, an array of narrow stripe semiconductor lasers is coupled together by a series of Y-junctions. This structure can be tuned to select the in-phase mode by ensuring that mode has the minimum loss.